1. Field of the Invention
The present invention relates to an optical code division multiplexing communication method using a time-spreading/wavelength-hopping code, an optical code division multiplexing communication system capable of implementing the optical code division multiplexing communication method, an encoding device constructing the optical code division multiplexing communication system, and a decoding device constructing the optical code division multiplexing communication system, and in particular, to chromatic dispersion compensation in optical code division multiplexing communication.
2. Description of the Related Art
FIG. 1 is a diagram for describing a principle of encoding and decoding in an optical code division multiplexing communication using a time-spreading/wavelength-hopping method. As shown in FIG. 1, an encoder 10 has a structure in which FBGs (Fiber Bragg Gratings) 10a, . . . , 10d of multiple wavelengths (for example, reflecting wavelengths are λ1, λ3, λ4, and λ2) are connected in series. When wavelength multiplexing pulse 30 corresponding to the reflecting wavelengths of the FBGs 10a, . . . , 10d is inputted to the encoder 10 via an optical circulator 34, a multi-wavelength optical pulse train including four optical pulses 31a, . . . , 31d of different wavelengths is produced in a spreading time determined by a code set in the encoder 10 (that is, structure of the encoder 10) from one wavelength multiplexing pulse 30 and is outputted to a transmission line 35 via an optical circulator 34. This multi-wavelength optical pulse train is an encoded optical signal. A decoder 20 of the same codes as the encoder 10 (that is, having a structure of handling the same codes as the encoder 10) has a series structure of FBGs 20d, . . . , 20a which is opposite to the series structure of the encoder 10 in which the FBGs 10a, . . . , 10d are connected in series. Hence, the decoder 20 has a group delay time characteristic which is opposite to the group delay time characteristic of the encoder 10. Therefore, when the encoded signal (optical pulses 31a, . . . , 31d) is inputted to the decoder 20 via an optical circulator 36, time-spread optical pulses of four wavelengths are aligned at the same timing, whereby wavelength multiplexing pulse 33 of auto-correlation waveform is produced and outputted via the optical circulator 36.
When the encoded signal is inputted to the decoder 20, if the codes agree with each other (the series structures of the multi-wavelength FBGs of the encoder 10 and the decoder 20 are related as object and mirror image), the relative time arrangement of the time-spread multi-wavelength optical pulses 31a, . . . , 31d is corrected to produce a wavelength multiplexing pulse 33 having an auto-correlation waveform. If the codes do not agree with each other (the series structures of the multi-wavelength FBGs of the encoder 10 and the decoder 20 are not related as object and mirror image), the relative time arrangement of the time-spread multi-wavelength optical pulses 31a, . . . , 31d is further spread to produce an cross-correlation waveform (not shown).
In general, the optical code division multiplexing communication using a time-spreading/wavelength-hopping code, as disclosed in the non-patent document 1 (Wei et al., “BER Performance of an Optical Fast Frequency-Hopping CDMA System with Multiple Simultaneous Users”, OFC2003, Technical Digest, Vol. 2, ThQ1, pp. 544-546), is characterized in that because an optical signal of a plurality of wavelengths spread in a time range (having a wide frequency band) is used as an encoded signal, as a transmission distance is longer and a transmission signal rate is faster, the optical signal is more susceptible to the chromatic dispersion of the transmission line. Because a transmission line formed of a standard SMF has a chromatic dispersion characteristic of approximately 17 ps/(nm·km), when an encoded optical signal is transmitted through the transmission line, the relative time arrangement between the multi-wavelength optical pulses constructing the encoded optical signal varies according to the transmission distance. Hence, even if a decoder of the same codes as an encoder is used, the auto-correlation waveform as shown in FIG. 1 cannot be produced (that is, the encoded optical signal cannot be excellently decoded). Therefore, when optical code division multiplexing communication using a time-spreading/wavelength-hopping code is applied to a transmission line having chromatic dispersion, if the chromatic dispersion is not compensated to a sufficiently smaller level with respect to the width of an optical signal pulse inputted to the encoder, an excellent auto-correlation waveform cannot be produced. However, when the chromatic dispersion of an individual transmission line is compensated by known means such as a chromatic dispersion compensating fiber, the communication system needs to be increased in size, which results in increasing transmission loss and costs.
As to the problem like this, to reduce the effect of the chromatic dispersion with ease, a non-patent document 2 (Iwamura et al., “FBG based Optical Code En/Decoder for long distance transmission without dispersion compensating devices”, OFC2004, Technical Digest, WK6) discloses a technology of compensating a delay time difference in a wavelength band (frequency band) in an optical encoded signal, of the effects caused by the chromatic dispersion of an FBG type decoder, by the construction of the FBG type encoder. According to this technology, it is achieved to transmit an encoded optical signal through the SMF of 40 km at a transmission rate of 10 Gbps. FIG. 2 is a diagram for describing an encoder and a decoder having a delay time compensating function in an optical code division multiplexing communication using a time-spreading/wavelength-hopping method. As shown in FIG. 2, when the encoded optical signal is transmitted through the SMF of 40 km, an optical pulse train (optical pulses 31a, . . . , 31d) immediately after produced by the encoder 10 is turned to an optical pulse train (optical pulses 32a, . . . , 32d) having the delay time difference between wavelengths increased by the transmission through the SMF. Therefore, a decoder 21 has a delay time characteristic in which a delay time characteristic of compensating delay time differences between wavelengths caused by the chromatic dispersion of the SMF transmission line is added to a delay time characteristic opposite to that of the encoder 10.
However, even if the decoder 21 shown in FIG. 2 is used, it is only a wavelength difference and a delay time difference caused by a transmission distance that are compensated, and the optical pulse spread in a time direction of individual optical pulse, which is caused by the chromatic dispersion of the fiber, cannot be compensated. Hence, as the transmission distance is longer, the widths of auto-correlation waveforms are more expanded and the optical pulses finally overlap neighboring optical pulses, thereby making it impossible to receive the optical signal. In this manner, even if the decoder 21 shown in FIG. 2 is used, it is only delay time differences between optical signal wavelengths that are compensated and the expansion of width of the individual optical signal pulse, which is caused by the dispersion slope of the transmission line, is not compensated. Therefore, the transmission distance cannot be more extended.
Further, a non-patent document 3 (Buryak et al., “Optimization of Refractive Index Sampling for Multichannel Fiber Bragg Gratings”, IEEE Journal of Quantum Electronics, Vol. 39, No. 1, pp. 91-98, January 2003) discloses a technology related to dispersion slope compensation but does not disclose a technology applied to the compensation of encoded waveforms degraded by the transmission line of the optical code division multiplexing communication of a time-spreading/wavelength-hopping method using an optical signal of a plurality of wavelengths for one communication channel.